Conventional adaptive modulation technology realizes fast and stable high-quality wireless transmission over a limited frequency bandwidth. Adaptive modulation technology adaptively controls the transmission rate determined by a transmission signal's modulation method, coding rate, etc., in accordance with the channel state, this channel state changing as the surrounding environment changes due to the passage of time or the wireless communication device's movement (see, for example, Patent Literature 1). For example, QPSK (Quadrature Phase Shift Keying), which has a low rate, is used for the modulation method when the channel state is poor, and 16 QAM (Quadrature Amplitude Modulation) or 64 QAM, which have a high rate, is used when the channel state is good. For the coding rate, when the channel state is poor, coding that has a strong error correction capability and a low coding rate is used, and when the channel state is good, coding that has a weak error correction capability and a high coding rate is used.
In recent years, development of wireless communication systems that take advantage of special multiplexing technology, using a plurality of antennas as a means for realizing next-generation high-speed, high-capacity communication, has progressed. The related technology is called the MIMO (Multiple Input Multiple Output) method, and by increasing the amount of multiplexing (the number of transmission streams), it is possible to increase communication capacity without increasing the frequency bandwidth.
In wireless communication using a single antenna, determining the transmission rate used in wireless communication from a transmitter to a receiver in accordance with the received signal strength level in the receiver makes the transmission rate an appropriate rate for the channel state. The transmission quality under the MIMO method, however, depends not only on the received signal strength level, but also on a channel matrix whose elements are estimated values of the channel characteristics for each channel. This is because, for the equalization processing under the MIMO method, it is necessary to separate the transmission streams.
For this reason, just because the received signal strength level under the MIMO method is high does not necessarily mean that separation of the original transmission streams can be carried out accurately. Therefore, when adaptive modulation technology is applied under the MIMO method, even if the transmission rate used in wireless communication from a transmitter to a receiver is determined in accordance with the received signal strength level, it is not necessarily true that the transmission rate is an appropriate rate for the channel state.
To determine the transmission rate used in wireless communication from a transmitter to a receiver, adaptive modulation technology that performs error detection using CRC (Cyclic Redundancy Check) for each transmission stream and determines the transmission rate used in wireless communication from a transmitter to a receiver based on the results of error detection has been proposed (see, for example, Patent Literature 2). An overview of this technology is now provided using FIG. 50.
In the transmitter 1000, each of the CRC bit adding units 10011-1001N adds a CRC bit sequence to the information bit sequence input in each unit. Next, in accordance with the coding rate and modulation method indicated by the receiver 2000, each of the coders/mappers 10021-1002N performs coding processing and modulation processing on the information bit sequence to which the CRC bit sequence was added. The transmission signal for the plurality of transmission streams is then transmitted from each of the antennas 10031-1003N.
In the receiver 2000, the channel estimation unit 2002 estimates each channel's SINR (Signal to Interference and Noise Power Ratio) from a signal received by each of the antennas 20011-2001K. The detection unit 2003 then divides and combines, from the received signal, the transmission signal for the plurality of transmission streams, based on the estimated SINR value for each channel as estimated by the channel estimation unit 2002. Next, the demappers/decoders 20041-2004N each perform demodulation processing and decoding processing on the transmission signal input from the detection unit 2003, and the CRC error detection units 20051-2005N each perform CRC error detection on the input bit sequence. Furthermore, the transmission rate determining unit 2006 determines the coding rate, modulation method, etc. to be used for wireless communication from the transmitter 1000 to the receiver 2000 for subsequent transmissions based on the estimated SINR value for each channel and on the results of CRC error detection for each transmission stream. The transmission rate determining unit 2006 then notifies the transmitter 1000 of the determined coding rate, modulation method, etc.
[Citation List]
    Patent Literature 1: Japanese Patent Application Publication No. 2005-244598    Patent Literature 2: Japanese Patent Application Publication No. 2006-14066